In making up, the operations of sewing pieces of woven or knitted fabric generally aim to sew two pieces together along one of their edges or to form a hem at the border of one piece, or again to fix a tape, a piece of elastic, a waistband etc along one edge of a piece. In order to attempt to save on labour and make gains in productivity, guidance systems have been proposed which move the woven or knitted fabric in the transverse direction upstream of the needle in order to constantly bring the area to be sewn facing the needle; these systems generally comprise wheels or caterpillars with rollers, disposed in the transverse direction upstream of the needle in the direction of driving, and which are driven in rotation in one direction or the other so as to be able to move the material transversely (at right angles to the direction of driving) in both directions in order to make suitable position corrections. However, these systems are little used since they result in imperfect sewing and frequent defects. This is because the flexible material driven by the presser foot level with the needle is braked upstream of the latter by the guidance system and has a tendency to roll on itself, to become folded or to wrinkle in a random fashion, which is incompatible with obtaining an even quality stitching; this defect is particularly serious for stretch or "soft mesh" knitwear which it is impossible to sew with these known systems; in the case of two pieces to be joined edge to edge; offsets of one edge with respect to the other occur, and joining imperfections which are prejudicial to the quality of the product produced. In addition, the majority of existing systems can be used only for materials with a defined thickness.
Subsequently (patents FR 2 585 683 and EP 0 216 644), a guidance device was proposed which comprises a spherical wheel coming into abutment against the material upstream of the presser foot and turning in a single direction of rotation in order to drive and guide the material upstream of the stitching area. This device has a radically different principle from the other prior systems since, in these patents, the spherical wheel is offset laterally, drives the material in the same direction as the presser foot, but is mounted so as to pivot about a vertical axis so as to be able to orient it by virtue of a control of the proportional analogue type in different driving directions enabling the material to be brought into the required position. In the device to which these patents relate, the spherical wheel is driven in rotation by friction by means of a roller which comes into abutment on one side thereof in the horizontal plane containing the axis of the wheel.
However, such a system comes up against a serious practical difficulty in implementation. To afford satisfactory driving of the spherical wheel by means of the roller, it is necessary for the surfaces of these components to roll without sliding against each other and therefore to have a sufficient coefficient of friction; when the spherical wheel pivots about the vertical axis in order to adopt another orientation, the abutment of the roller on this wheel describes a segment of a great circle and this movement is transverse with respect to the roller (parallel to the axis of the roller and perpendicular to the tangential velocity), so that the friction of the two surfaces has a tendency to oppose the movement. Under these circumstances, the pivotings of the spherical wheel in order to modify its orientation require the involvement of high forces, must be slow compared with the speeds of rotation of the roller and of the wheel, and produce rapid wear on the surfaces which requires frequent changes of these components.
In order to mitigate these drawbacks, EP-A-0 468 578 next proposed a guidance device on which the roller is positioned above a rotating component for adjustment by driving the material (elastic sphere) so as to come into abutment with the upper area of the component turning in the opposite direction to the area of contact of the component with the material, the area of abutment of the roller with the rotating component being situated substantially on the vertical axis about which the pivoting of the said rotating component takes place. Thus the material is longitudinally driven by the rotating component, in a permanent fashion, at a point of contact situated on the driving straight line upstream of the presser foot, and the position corrections are made by orienting the direction of this driving in a suitable fashion.
This system gives satisfaction but is nevertheless limited to the guidance of pieces parallel to reference lines whose radius of curvature is large, notably greater than approximately 25 cm. Thus this system is not in practice sufficiently rapid and effective for producing automatic stitching parallel to lines having portions with a very pronounced curvature (radius of curvature less than 25 cm), or even angular. Such stitches must therefore still be produced manually by operators who guide the material in front of and upstream of the needle.
It should be noted that, in the known guidance devices, up to now attempts have been made to automate the action of the operator by guiding the material immediately upstream of the needle. Thus, in all the above-mentioned guidance devices with a spherical rotating component, the position of the material is adjusted by varying the angle of orientation of the axis of rotation of the rotating component in one direction or the other.
In addition, it has up till now been considered inevitable, in these prior devices, for the guidance of the material not to be able to be provided continuously between the adjustment component and the presser foot, that is to say over a distance which may extend in practice over several centimeters.
Moreover, FR-2 392 909 describes a device comprising several rollers rotating about a common fixed axis perpendicular to the driving direction and passing through the needle. The different motors of the different rollers are controlled by an analogue control which applies to them voltages such that the centre of pivoting applied by all these rollers coincides with the centre of curvature of the border of the piece of cloth being stitched. This device is conceivable only with wide pieces and results in practice in many phenomena of unwanted folds, notably in the case of flexible or stretch fabrics. In addition, it is complex, extremely tricky, if not impossible, to adjust satisfactorily, and requires adjustments at each change of fabric and/or shape of piece.
U.S. Pat. No. 4,813,364 describes an endless band disposed laterally in contact along a linear contact surface parallel to the direction of driving. Such a device is completely ineffective in practice since the band in contact with the fabric prevents any pivoting or causes significant folds in the fabric.